There are various schemes for consolidating metallic powders. Among the more common methods are hot isostatic pressing ("HIP"), hydrostatic pressing, explosive forming, slip casting, can extrusion and injection molding. Each technique has its advantages and disadvantages. The disadvantages generally include complex and expensive equipment and limited final configurations.
The instant invention, however, is concerned with powder metallurgy ("PM") slurry techniques such as extrusion and rolling. The equipment is essentially conventional, widely available and does not call for exceedingly great care to operate successfully.
In brief, metallic powder is mixed with a water soluble binder, lubricant, and water to form a thick slurry. The slurry is then introduced into an extrusion press, rolling mill, or injection molding die to produce a desired shape. The resulting product is dried and sintered. Key benefits of this processing route are improved yield and resultant cost savings.
Unfortunately, the resulting product may have poor density and, therefore, unacceptable working characteristics. In order to improve the formability properties, the density of the object in most cases must be high. Although low density is not always associated with low formability, given identical powder characteristics, increased density will result in improved formability.
Another benefit of high density is that the piece can tolerate a more severe forming operation. At very low density levels (70-80% dense), the material can only be consolidated by complete compressive operations such as HIP. At higher density levels (80-90%), the piece can be cold formed (or hot formed under atmosphere) by partially compressive operations such as the reducing or rolling. With 90% density or better, the piece can be hot worked in air as the porosity is not interconnected and internal oxidation is not a problem. At 95% density or better, the piece can tolerate some tensile operations such as hot rolling or drawing. At 99% dense or better, the piece can be treated as a wrought material. To summarize, density increases can be associated with improved formability and an increasing diversity of available forming operations.
Moreover, the orientation of the voids within the product is paramount. Spherical voids are to be avoided since they tend to lower the strength of the product. Rather, irregular voids are desirable inasmuch as they boost the strength of the object.
Other researchers have noted the effect of boron containing additions on powder alloys. Firstly, U.S. Pat. No. 3,704,508 outlines the CAP (consolidated at atomospheric pressure) process. Here, metallic powders are mixed with a boric acid-methanol solution, sealed and sintered to a fully dense piece. Secondly, U.S. Pat. No. 4,407,775 reveals a method to consolidate metallic powders by the addition of lithium tetraborate. The process utilized in this reference is identical to that of the CAP process. Thirdly, U.S. Pat. No. 4,113,480 discloses a method for injection molding of powders where a boric acid-glycerin mix is used to promote mold release and densification. Lastly, U.S. Pat. No. 4,197,118 relates to a method of binder removal before sintering.